1. Field of the Invention
The present invention relates to an apparatus for painting a desired paste-pattern on a substrate by moving the substrate and a paste discharging nozzle relatively to each other and applying paste to the relatively moving substrate.
2. Description of Prior Art
Japanese Unexamined Patent Publication No. 2-52742 discloses an art in which a desired pattern of a resistance is formed on a substrate by discharging paste of the resistance from a paste discharging nozzle onto the substrate which is being moved relatively to the paste discharging nozzle. In such a conventional art, if the clearance between the substrate and the nozzle varies during a pattern drawing operation, the drawn paste pattern loses a desired sectional shape. Therefore, to maintain a predetermined clearance between the substrate and the nozzle, a sensor is employed to monitor the clearance therebetween. Preceding the nozzle moving relatively to the substrate, the sensor detects undulation (flatness) of a portion of the substrate to which the paste is applied. A predetermined clearance between the nozzle and the substrate is maintained based on the detected undulation, for example, by shifting the nozzle accordingly.
In the conventional art, such a sensor is fixed to a nozzle mounting means. If a paste pattern to be drawn is of one-way straight line so that the direction of the relative movement of the nozzle with respect to the substrate does not vary, a predetermined clearance between the nozzle and the substrate can be maintained since the sensor correctly detects undulations of the surface of the substrate. However, if a paste pattern to be drawn includes a curved portion or a rectangular portion so that the direction of the relative movement thereof varies, the sensor does not always precede the nozzle as viewed from the substrate moving with respect to the sensor and the nozzle. The sensor may be at a lateral side of or behind the nozzle according to the direction of the relative movement thereof which varies. Therefore, as the direction of the relative movement of the nozzle varies, a sensor's measuring point on the substrate, which is slightly spaced from a paste application point on the substrate, may come close to or move across a line of the paste pattern drawn even if the nozzle tip is not close to or facing the drawn line of the paste. In such a case, the pattern of the applied paste causes an error in the measurement of the clearance between the nozzle tip and the substrate. The detection of undulations of the substrate surface fails, resulting in an inaccurate measurement of the clearance between the nozzle and the substrate.
If the clearance between the nozzle and the substrate is not accurately measured, the drawn paste pattern will have varying sectional shapes because the amount of paste discharged from the nozzle per unit of time is constant. For example, at a position where the clearance between the substrate and the nozzle is close, the line of the paste pattern becomes wider. In a place where the clearance is larger, the past line becomes narrower. The height of the drawn paste pattern also varies accordingly. Thus, the paste pattern fails to have consistent sectional shapes.